Vehicle hydraulic system with pump speed control

ABSTRACT

A lift truck is disclosed with a changeable speed electric motor for driving a single positive displacement pump to supply the power steering and various work handling hydraulic load circuits. Manually actuable selector means are operatively connected with the load circuits and a speed range control means is operatively connected with the selector means for energizing the electric motor in different speed ranges according to the actuation of the selector means. A priority demand valve is connected between the pump and the respective load circuits and supplies fluid to the load circuits, giving priority to the power steering circuit. The speed range control means includes speed regulating means for the electric motor. The motor is energized from the battery and the regulating means includes a pulsing circuit with a motor current feedback to vary the mark/space ratio of the pulses produced by the pulsing circuit.

FIELD OF THE INVENTION

This application discloses subject matter in common with U.S. Ser. No.400,769 filed on July 22, 1982 which is a continuation of U.S. Ser. No.291,673, filed on Aug. 10, 1981 (now abandoned) entitled "VEHICLEHYDRAULIC SYSTEM WITH SINGLE PUMP" by Willard L. Chichester and assignedto the same assignee as this application.

This invention relates to work vehicles such as lift trucks; moreparticularly, it relates to an improved hydraulic system for selectivelyenergizing the hydraulic work circuits such as the vehicle powersteering circuit and the load handling power actuator circuits.

BACKGROUND OF THE INVENTION

It has been a common practice to provide lift trucks with two or morehydraulic pumps for supplying the different hydraulic circuits requiredfor the various load handling and vehicle operating functions. Forexample, one pump having a relatively high flow capacity is used forsupplying the load handling actuator circuits, such as the lift and tiltcircuits for the extendable upright. A smaller pump is provided tosupply the hydraulic circuit of the vehicle power steering It is alsoknown practice to provide additional pumps for other hydraulic circuitson the vehicle. In some instances, two or more pumps are connected inseparate and independent hydraulic circuits and in other instances, thepumps are connected in common circuits. Typically, in a lift truck, theflow and pressure requirements of the power steering circuit are muchlower than the requirements of the lift circuit. When the requirementsof the various hydraulic circuits are met by using plural pumps, thepumps must be sized according to the maximum flow requirements of therespective circuits. Each additional pump and its attendant drivearrangement, hydraulic and electrical circuit components requiresadditional space and adds considerable weight to the vehicle.Accordingly, it is desirable to eliminate the need for plural hydraulicpumps and the duplication of circuit components in the hydraulic systemof a work vehicle.

It has not been practical to use a single pump such as the large pumprequired for the lift circuit, which may have a by-pass pressure of 3000PSI, for supplying the power steering circuit which needs only fivegallons per minute at 1000 PSI. Certain load handling circuits such as atilt circuit for the upright and an auxiliary circuit for a side shiftermay be operated from the large pump of the lift circuit because each useis of short duration. However, other load handling circuits such asauxiliary circuits for clamps and rotaters are not desirably suppliedfrom the pump for the tilt circuit because such devices are usedfrequently and for prolonged periods. In the Chichester U.S. Pat. No.3,568,868 a single pump supplies fluid to both the tilt circuit and thelift circuit of the extendable upright. For this purpose, a flowpriority valve is connected between the outlet of the pump and the inletports of the tilt and lift control valves so that the flow requirementof the tilt actuators is always supplied. A one way valve connects thetilt circuit to the lift circuit so that the entire flow from the pumpcan be supplied to the lift circuit. In this arrangement, a 48 voltbattery may be substituted for a 32 volt battery without adverseconsequence even with increased pump output because the flow priorityvalve regulates the flow to the tilt actuators and the excess flow isdirected to the lift circuit.

In the Williams et al U.S. Pat. No. 3,964,260, a single hydraulic pumpis utilized to supply fluid to the power steering circuit and also tothe lift and tilt circuits of the extendable upright. The pump hassufficient capacity to meet the maximum flow requirements and any excessfluid is diverted to an accumulator. The steering circuit is connectedto the accumulator so that sufficient flow will be available in theevent of need for emergency steering.

A general object of this invention is to provide an improved hydraulicsystem for energizing the power steering and load handling hydrauliccircuits of a work vehicle.

SUMMARY OF THE INVENTION

In accordance with this invention, a work vehicle is provided with achangeable speed electric motor for driving a single positivedisplacement pump to supply the hydraulic flow requirements of at leasttwo hydraulic load circuits. First and second manually actuable selectormembers are operatively connected with first and second load circuits,respectively. The pump is adapted to supply a greater hydraulic flow, asrequired by the second load circuit, when the electric motor is operatedin a high speed range and to supply a lesser flow, as required by thefirst circuit, when the motor is operated in a low speed range. Apriority demand valve is connected between the outlet of the pump andthe respective inlets of the first and second circuits. The selectormembers are actuated either separately or together to select operationof one or both load circuits at a time. Speed range control means isoperatively connected with the selector means for energizing theelectric motor for operation in the high speed range when the secondselector means is actuated and for energizing the electric motor in thelow speed range when only the first selector member is actuated.Accordingly, the priority demand valve supplies fluid to the firstcircuit when the motor is operated in either the low speed or high speedrange.

Further, according to the invention, the vehicle may be provided withthree or more hydraulic load circuits, with a third manually actuableselector member operatively coupled with the third load circuit. Thethird load circuit has a hydraulic flow requirement intermediate that ofthe first and second circuits. The speed range control means isoperatively connected with the third selector means for energizing theelectric motor for operation in an intermediate speed range when thethird selector means is actuated and the pump is adapted to supply thehydraulic flow required by the third load circuit. The priority demandvalve is connected between the outlet of the pump and the inlet of thethird load circuit and is operated so that it supplies fluid from thepump to the first load circuit when the electric motor is operated inthe low speed range and above the low speed range. It is operated sothat it supplies fluid to the first and second load circuits when themotor is operated in the high speed range and to the first and thirdload circuits when it is operated in the intermediate speed range.

Further, according to this invention, the speed range control meansincludes switching means actuated by the manually actuable selectormembers for selecting the speed range for the electric motor.

Further, according to this invention, the speed range control meansincludes regulating means for maintaining the speed of the electricmotor within predetermined limits during operation in at least one ofthe speed ranges.

Further, according to the invention, the motor is energized by a batteryand the regulating means includes means for applying DC pulses ofvariable mark/space ratio to the motor and means responsive to the motorcurrent for adjusting the mark/space ratio to regulate the motor speedwithin the predetermined limits.

A more complete understanding of this invention may be obtained from thedetailed description that follows.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a lift truck embodying the subjectinvention;

FIG. 2 is a perspective view of certain manual controls for use by thedriver;

FIG. 3 is a block diagram of the hydraulic system of this invention;

FIG. 4 is a diagram of the electrical system;

FIG. 5, taken on line 5--5 of FIG. 2, shows the lift control lever andassociated speed selector switch;

FIG. 6, taken on line 6--6 of FIG. 2, shows the auxiliary control leverand associated speed selector switch;

FIG. 6a shows the auxiliary control lever in an alternate position;

FIG. 6b is a view taken on lines 6b--6b in FIG. 6;

FIG. 7 shows the voltage control circuit for the drive motor of thepump;

FIG. 8 shows certain details of the circuit of FIG. 7;

FIGS. 9, 10 and 11 are graphical representations of operationalcharacteristics of the drive motor;

FIG. 12 is a schematic diagram of the hydraulic system;

FIG. 13 shows a schematic diagram of the hydraulic system energized inthe steering mode;

FIG. 14 shows a schematic diagram of the hydraulic system energized inthe lift and steering mode combined;

FIG. 15 shows a schematic of the hydraulic system energized in the tiltmode; and

FIG. 16 shows a schematic diagram of the hydraulic system energized inthe auxiliary mode.

MODE FOR CARRYING OUT THE INVENTION

General Description Of The System

Referring now to the drawings, there is shown an illustrative embodimentof the invention in the hydraulic system of a work vehicle, specificallya lift truck. It will be appreciated as the description proceeds, thatthe invention may be employed in other types of work vehicles and isuseful in other applications.

Referring now to FIG. 1, there is depicted an electric lift truck 10which is supported on a pair of traction wheels 12 and a pair ofdirigible wheels 14 for direction control of the vehicle. The vehicle isprovided with a driver's station including a driver's seat 16 and asteering wheel 18 which is coupled with the dirigible wheels 14 throughan hydraulic power steering system which will be described subsequently.The lift truck includes an extendible upright 20 which is mounted on thevehicle frame and includes a carriage 22 with load engaging forks 24.The carriage 22 is movably mounted on the upright 20 and is raised orlowered by an hydraulic motor 26. The upright 20 is mounted adjacent itslower end on the vehicle frame for pivotal motion about a horizontalaxis and is tilted in a forwardly or rearwardly direction by a pair ofhydraulic motors 28. In addition, the lift truck may be provided with anauxiliary load handling device (not shown in FIG. 1), such as a sideshifter, clamp or rotator.

FIG. 2 depicts the lift truck with the manual controls accessable at thedriver's station for driving the vehicle and operation of the loadhandling functions, including the movement of the carriage 22 and anyauxiliary load handling device that may be added. The manual controlsfor the load handling functions include a lift control lever 32, a tiltcontrol lever 34, and an auxiliary control lever 36. These controllevers will be described in detail subsequently in relation to thecontrol functions in the hydraulic system of this invention. A driveselector lever 38 is mounted on the steering column 40 and is coupledwith a controller 42 which provides starting and speed control of thetraction motor. The controller 42 is also adapted to control the parkingbrake (not shown in FIG. 2) in accordance with the state of a seatoccupancy detector 44. The controller 42, per se, forms no part of thepresent invention; it does, however, control the actuation of certainswitches which affect the operation of the hydraulic system. Suffice itto say, that the functions of the controller 42 are represented in thediagram of FIG. 4 which will be described presently. It is notedhowever, that the drive selector lever 38 is coupled by suitable linkage46 with the controller 42 for the selection of neutral, forward, reverseor park modes of operation of the traction motor. Also, the seatoccupancy detector 44 includes suitable linkage 48 coupled between theseat 16 and the controller 42. An accelerator pedal 52 is coupledthrough suitable linkage 54 to the controller 42 for starting and speedcontrol of the traction motor. The driver's station additionallyincludes the foot brake pedal 56 for operation of a service brake (notshown in FIG. 2) for the vehicle.

FIG. 3 is a block diagram of the hydraulic system of this invention. Thehydraulic system comprises a single hydraulic pump 60 of the positivedisplacement type. The prime mover for the pump is an electric motor 62which is a DC series motor. The system further comprises a steering unit64 and a main valve 66. Hydraulic fluid is supplied from the pump 60 tothe steering unit 64 and through the steering unit to the main valve 66.The steering unit 64 includes a steering control valve (not shown inFIG. 3) which supplies fluid to the hydraulic motor or steering cylinder68 for power steering. The steering unit also supplies fluid to thehydraulic motor or brake cylinder 72 for the parking brake and includesa return to a hydraulic reservoir or tank 74. The main valve 66 suppliesfluid to the hydraulic motor or lift cylinder 26 for the carriage 22.The main valve 66 also supplies fluid to the hydraulic motors or tiltcylinders 28 and 28' for tilting the upright 20. Also, the main valve 66supplies fluid to an auxiliary cylinder 368. Fluid from the main valveis returned to the tank 74 through a return line filter 76.

FIG. 4 depicts the electrical circuits, in block diagram, for energizingthe drive motor 62 for the hydraulic pump 60 and for energizing thetraction motor 78. It is noted that the voltage from the battery 80 issupplied through a key switch 82 to the controller 42 and thencedirectly to the pump motor 62. The energizing circuit for the pump motor62 is completed through a voltage control circuit 86. The pump 60 isconnected with the hydraulic system 100. The battery voltage is appliedfrom the controller 42 through the motor control circuits 84 to thetraction motor 78. The controller 42 includes switching circuits whichare controlled by the drive selector 38, the seat occupancy detector 44and the accelerator pedal 52 in a manner which will be describedpresently.

The controller 42, as shown in FIG. 4, includes a start switch 88 whichis connected in series with the key switch 82 between the battery 80 andthe motor control circuit 84. The start switch is actuated by movementof the accelerator pedal 52; the switch is open with the acceleratorpedal in its retracted position and the switch is closed during aninitial increment of movement when the accelerator pedal is depressed. Aspeed control member 90 of the inductive type is also coupled with theaccelerator pedal 52 and a speed control signal is transmitted from themember 90 to the motor control circuits 84. Direction control for thetraction motor 78 is provided by a forward control switch 92 and areverse control switch 94. The forward and reverse control switches areconnected in parallel with each other and in series with the startswitch 88 between the battery and the motor control circuit. Thus, thetraction motor 78 will be energized through the motor control circuits84 when the key switch 82 and the start switch 88 are closed with eitherthe forward control switch 92 or the reverse control switch 94 closed.The controller 42 also includes a pump control switch 96 which isserially connected with the key switch 82 between the battery 80 and thepump motor 62. Accordingly, when the pump control switch 96 is openedthe pump motor 62 is turned off and the fluid to the hydraulic system100 is cut off.

The forward control switch 92, reverse control switch 94, and the pumpcontrol switch 96 are selectively actuated by the drive selector lever38 which may be placed in park position, forward position, neutralposition or reverse position. When the drive selector lever is in theneutral position, as depicted in FIG. 4, both the forward switch 92 andthe reverse switch 94 are open and hence, the traction motor is turnedoff. When the selector lever 38 is in the forward position, the forwardswitch 92 is closed and the reverse switch 94 is open and hence, thetraction motor will be energized when the start switch 88 is closed.Similarly, when the selector member 38 is in the reverse position, thereverse control switch 94 is closed and the forward control switch 92 isopen and hence, the motor will be energized when the start switch 88 isclosed. When the selector member 38 is in the park position, the forwardcontrol switch 92 and the reverse control switch 94 are open and thepump control switch 96 is open. This turns off the traction motor 78 andthe pump motor 62 and hence, fluid to the hydraulic system 100 is cutoff.

The selective actuation of the forward control switch 92, reversecontrol switch 94 and the pump control switch 96 is also controlled bythe seat occupancy detector 44. The occupancy detector is operative,when the selector lever 38 is in the neutral position to cause the pumpcontrol switch 96 to open and thereby turn off the pump motor 62 whenthe driver dismounts from the seat. Also, the detector 44 is operative,when the selector lever 38 is in either the forward or reverse position,in response to the driver dismounting from the seat, to cause theforward control switch 92 and the reverse control switch 94 to open anddeenergize the traction motor and to cause the pump control switch 96 toopen and deenergize the pump motor.

In summary, the electrical circuit of FIG. 4 as just described, isoperative to deenergize the traction motor 78 unless the key switch 82,the start switch 88 and one of the forward or reverse control switches92 and 94, respectively, are all closed. Also, it is operative todeenergize the pump motor 62 unless the key switch 82 and the pumpswitch 96 are both closed; the pump switch 96 is closed only when thedriver's seat is occupied and when the drive selector lever 38 is inforward, reverse or neutral position. The pump switch 96 is open whenthe drive selector lever 38 is in the park position regardless ofdriver's seat occupancy. As a result of this control, hydraulic fluid tothe hydraulic system 100 is cut off unless the driver occupies the seatand the drive selector lever is in forward, reverse or neutral.

The hydraulic system 100 of this invention, is subject to manual controlin the selection of different work functions to be performed. Thehydraulic system 100 includes the power steering circuit for thedirigible wheels 14 of the work vehicle, a power lift circuit for thework engaging carriage 22, a power tilt circuit for the upright 20 and apower actuation circuit for the auxiliary load handling device (notshown in FIG. 1 or 2). The manual control of the hydraulic system 100 isexercised by the steering wheel 18 for power steering and by therespective hand levers 32, 34 and 36 for the lift, tilt and auxiliaryfunctions. For this purpose, the steering wheel 18 is coupled with thesteering valve 102 (see FIG. 3) for bi-directional actuation of thevalve in response to steering wheel motion. The lift control lever 32 iscoupled with the lift valve 104 and the tilt control lever 34 is coupledwith a tilt control valve 106. Similarly, the auxiliary control lever 36is coupled with an auxiliary control valve 108. The control levers 32,34 and 36 will be described in greater detail below.

Detailed Description Of The Control Levers

The lift control lever 32 is shown in FIG. 5. It is adapted to actuatethe lift control valve 104 which is a part of the hydraulic system 100;it also is adapted to actuate a speed control switch 112 which isoperatively connected with the hydraulic system 100 in a manner whichwill be described subsequently. The lift control valve 104 is preferablya spool valve of the open center type having an operating stem 114. Thestem is axially movable in either direction from a neutral position toselect either the lift mode or lower mode of operation. The lift valve104 will be described in greater detail subsequently in connection withthe hydraulic circuit 100. The control lever 32 is pivotally mounted ona shaft 116 for pivotal motion in the fore and aft directions by thevehicle driver. The lever 32 is provided with a slot 118 which receivesa pin 122 extending through the operating stem 114 of the valve.Accordingly, when the control lever 32 is pivoted in the rearwarddirection (phantom lines) the stem 114 of the valve is raised from theneutral position and the valve is operated in the lift mode. When thehand lever 32 is rotated in the forward direction, the stem 114 islowered and the valve is operated in the lower mode. For the purpose ofactuating the switch 112, the control lever 32 is provided with an arm124 which actuates a push rod 126. The switch 112 comprises switchcontacts 128 which are biased by a spring 132 toward a closed position.A switch plunger 134 is adapted, when depressed, to hold the switchcontacts 128 in the open position. The push rod 126 is mounted in thecontrol lever housing 136 for reciprocal motion and is spring loaded bya bias spring 138 in the upward direction. With the lever 32 in theneutral position, the upper end of the push rod 126 engages the lowerface of the arm 124 and the lower end of the push rod 126 engages thelower face of the switch plunger 134. Thus, when the control lever 32 ispivoted rearwardly the push rod 126 is depressed against the resistanceof bias spring 138 and the lower end of the push rod moves away from theswitch plunger 134 allowing the switch contacts 128 to close under theinfluence of the spring 132. When the control lever is pivoted forwardlyfrom its neutral position the arm 124 moves away from the push rod 126and the push rod retains its position which holds the switch plunger 134depressed and the switch contacts 128 in the open position. In summary,with the lift control lever 32 in its neutral position, the lift controlvalve 104 is in its neutral position and the contacts of switch 112 areopen. When the control lever 32 is pivoted in the lift direction(rearwardly) the lift control valve 104 is in the lift mode and thecontacts of switch 112 are closed. When the control lever is pivoted inthe lower direction (forwardly) the lift control valve 104 is in itslower mode and the contacts of switch 112 remain open. The speed controlswitch 112 is connected with a control circuit for the pump motor 62which will be described subsequently.

The tilt control lever 34 is of the same construction as the liftcontrol lever 32 as just described with reference to FIG. 5. The tiltcontrol lever 34 controls the position of the tilt control valve 106(see FIG. 3); however, there is no speed control switch associated withthe tilt control lever 34. The valve 106 is preferably an open centerspool valve and its connection in the hydraulic circuit 100 will bedescribed subsequently. When the tilt control lever 34 is in its neutralposition, the tilt control valve 106 is in its neutral position. Whenthe control lever 34 is pivoted in the tilt-forward direction(forwardly) the tilt control valve 106 is in its tilt-forward mode. Whenthe control lever 34 is pivoted in the tilt-backward direction(rearwardly), tilt control valve 106 is in its tilt-backward mode. Theconnection of the tilt control valve 106 in the hydraulic circuit 100will be described subsequently.

The auxiliary control lever 36 is shown in FIG. 6a and 6b. This controllever is adapted to actuate the auxiliary control valve 108 which ispreferably a spool valve of the open center type. The control lever alsocontrols the actuation of an intermediate speed control switch 144 whichis of the same construction as switch 112 described above.(Corresponding parts of switches 112 and 144 have the same referencecharacters except that a prime symbol is added to the referencecharacters of switch 144.) The auxiliary control lever 36 is mounted onshaft 116 and is secured thereto for rotation of the shaft when thelever is rotated forwardly or rearwardly from its neutral position. Thecontrol lever 36 is provided with a slot 146 which receives a pin 148extending through the operating stem 152 of the auxiliary control valve108. Accordingly, when the control lever 36 is pivoted forwardly fromits neutral position, the control valve 108 is operated in its forwardmode and when the hand lever is rotated rearwardly, the valve 108 isoperated in its rearward mode. The control lever 36 is adapted tooperate the intermediate speed control switch 144 so that the switch isopen with the lever in the neutral position and is closed when the leveris moved either forwardly or rearwardly from the neutral position. Forthis purpose, push rod 168 is reciprocally mounted in the housing 136and is biased upwardly against the arm 166 by the bias spring 172. Anintermediate lever 154 is mounted on the shaft 116 and is fixed theretoby a pin 156 for rotation with the shaft and with the control lever 36.A switch operating lever 158 is pivotally mounted on the intermediatelever 154 by a pivot pin 162 so that the switch operating lever 158 isfree to rotate relative to the control lever 36 about the pivot pin 162,clearance from the shaft 116 being provided by a slot 164 in the lever158. An arm 166 of the lever 158 is adapted to engage the push rod 168and thrust the push rod downwardly when the switch actuating lever 158is rotated in a clockwise direction. A shoulder 172 on the lever 158overhangs an abutment 174 which is fixed relative to the housing 136.When the control lever 36 is pivoted rearwardly (FIG. 6, phantom lines)the shaft 116 rotates therewith and the intermediate lever 154 which isconnected therewith by the pin 156 is caused to rotate in a clockwisedirection. Thus, the pivot pin 162 carries the left end of lever 158upwardly causing it to pivot about shaft 116. This rotates the arm 166downwardly against the push rod 168 which allows the spring 132' toactuate the switch contacts 128' to the closed position. When the handlever 36 is pivoted forwardly (phantom lines, FIG. 6a) the intermediatelever 154 pivots therewith and, through the pin 162, carries the leftend of lever 158 downwardly so that the shoulder 172 engages theabutment 174. This causes the abutment 174 to serve as a fulcrum for thelever 158 and the downward thrust of the pivot pin 162 thereon causesthe right hand end of the lever 158, i.e. the arm 166, to rotatedownwardly. The downward motion of the arm 166 depresses the push rod168 and the switch 144 is closed.

The connection of the intermediate speed control switch 144 in the motorcontrol circuit will be described subsequently. Similarly, the auxiliarycontrol valve 108 and its connection in the hydraulic circuit 100 willbe described subsequently.

Pump Speed Control

According to this invention, the hydraulic pump 60, which is a positivedisplacement pump, is operated at different speeds according to the flowdemand which varies with the operating mode of the hydraulic system.When a light load on the hydraulic system is selected, a low motor speedis adequate. For example, in a typical lift truck hydraulic system thepower steering circuit requires a maximum of about five gallons perminute; with a positive displacement pump which is sized to deliver 20gallons per minute at full speed of 1800 RPM, a pump speed of 800 RPMwould be adequate for power steering. Similarly, for other operatingmodes, such as tilt of the upright, a low pump speed is adequate. For aheavy load on the hydraulic system, such as that imposed by operation inthe lift mode either alone or with simultaneous operation in thesteering or tilt mode, operation of the pump will be required at itsmaximum rated speed, such as 1800 RPM. For operation of the hydraulicsystem with an intermediate load, such as that imposed by operation inan auxiliary mode, for example a load handling clamp, intermediate flowis required and would be obtained at an intermediate speed, such as 1200RPM. Accordingly, the operating speed range of the pump motor isdetermined by the selection of the operating mode of the hydraulicsystem. In the illustrative embodiment, the pump motor is operated in alow speed range when the hydraulic system is operated in the powersteering or tilt mode or a combination thereof. It is operated anintermediate speed range for operation in an auxiliary mode or thecombination thereof with the tilt and steer modes. The pump is operatedin a high speed range for the lift mode and the combination thereof withany of the other modes. A motor speed control circuit will be describedwith reference to FIGS. 7 through 11.

As shown in FIG. 7, pump 60 is driven by a series DC motor 62 having aseries field winding 182. The motor is energized from the battery 80through a voltage control circuit 86. The voltage control circuit isadapted to regulate the motor speed within low, intermediate and highspeed ranges according to the operating mode of the hydraulic circuit.For this purpose, the voltage control circuit 86 is a thyristor-typepulsing circuit having presettable means for determining the mark/spaceratio for the different speed ranges and being provided with a feedbackmeans for changing the mark/space ratio as a function of motor current.Such a motor control circuit is disclosed in Morton et al U.S. Pat. No.4,119,898 granted Oct. 10, 1978.

The voltage control circuit 86 comprises a silicon control rectifier(SCR) 184 connected in series with the motor 62. A current sensingresistor 186 is connected in series with the SCR and develops a feedbackvoltage corresponding to the value of motor current. A pulse generator188 supplies a pulse train to the gate of the SCR 184 which controls theeffective motor supply voltage in accordance with the mark/space ratioof the pulse train. A mark/space demand signal circuit 190 produces ademand signal which is applied to the pulse generator 188 and determinesthe mark/space ratio of the pulse train. The value of the demand signalproduced by the demand signal circuit 190 is modified by the demandmodifying circuit 192 in accordance with the value of the feedbackvoltage supplied from the sensing resistor 186. In operation of thevoltage control circuit 86, the demand modifying circuit 192 produces anoutput signal which increases in magnitude as a direct function of thefeedback voltage from the resistor 186, and hence as a direct functionof motor current. The mark/space demand signal circuit 190 produces ademand signal which increases from a minimum value when the motorcurrent is zero to a maximum value when the motor current reaches apredetermined value I₀. This causes the pulse generator 188 to produce apulse train having a mark/space ratio which increases from apredetermined minimum value to a maximum value in correspondence withthe demand signal.

FIG. 8 shows certain details of the demand signal circuit 190 and itsinterconnection with the pulse generator 188. The output of the demandsignal circuit 190 is connected with the input of the pulse generator188 through a series connection of the key switch 82 and the pump switch96, which were described with reference to FIG. 4. Accordingly, unlessboth the key switch and the pump switch are closed there is no demandsignal input to the pulse generator 188 and the SCR is turned off andthe motor 62 cannot be energized. The demand signal circuit 190 isprovided with a speed range selection circuit 194 which comprises anexternal resistance switching loop for the demand signal circuit. Theloop includes a resistor 196 which is connected in parallel with thespeed control switch 144 and in series with the speed control switch112. The speed control switch 112 is operated by the lift control lever32, as described with reference to FIG. 5, and the speed control switch144 is operated by the auxiliary control lever 36 as described withreference to FIG. 6. When both switches 144 and 112 are open, i.e. withboth control levers in the neutral position, the resistor 196 isswitched out of the circuit and the demand signal circuit 190 produces ademand signal corresponding to the low speed range. When the switch 112is closed by actuation of the auxiliary control lever, the demand signalcircuit 190 produces a demand signal corresponding to the intermediatespeed range. When the switch 144 is closed by actuation of the liftcontrol lever, the demand signal circuit 190 produces a demand signalcorresponding to the high speed range. In this latter condition, thefull battery voltage is applied to the pump motor.

The operation of the pump motor is depicted by graphical representationin FIGS. 9, 10 and 11. In FIG. 9, curve A shows the mark/space ratio asa function of motor current for the low speed range of operation. Notethat the mark/space ratio varies directly with motor current from zerocurrent up to a predetermined current of I₀ at which point it reaches amaximum value which remains constant for higher values of motor current.Similarly, curve B represents the mark/space ratio as a function ofmotor current for the intermediate speed range. It is similar to curve Awith higher values of mark/space ratio. In FIG. 10, curve C representsmotor speed as a function of motor current for the low speed range ofoperation and similarly curve D is the speed curve for the intermediatespeed range. FIG. 11 shows the motor voltage as a function of motorcurrent; curve E represents the low speed range and the curve Frepresents the intermediate speed range of operation. When the motor isoperated in either speed range, the feedback in the voltage controlcircuit 86 tends to maintain the motor speed substantially constant eventhough the load on the motor changes. This results because a change inload is reflected by the motor current and, as shown in FIG. 9, themark/space ratio changes as a direct function of motor current.Increased load produces a higher mark/space ratio and hence a highervoltage is applied to the motor. As shown in FIG. 10, the motor speedremains almost constant when an increase in motor current is accompaniedby a certain increase in motor voltage. Thus, when the speed selectorcircuit 194 is set for either the low speed range or the intermediaterange, the selected speed is maintained within a given range of speedvalues by the voltage control circuit 86 even though the load on themotor is changed. Accordingly, for either speed range, the pump 60produces a flow which is maintained within a given range of flow valuesregardless of changes of load on the motor by reason of the differentoperating conditions of the hydraulic circuit 100, as will be describedbelow.

The Hydraulic System

General

A schematic diagram of the hydraulic system 100 is shown in FIG. 12. Ingeneral, the system comprises the positive displacement pump 60, thesteering unit 64, and the main valve 66. The system also includes thehydraulic reservoir or tank 74 and a return line filter unit 76.Additionally, the hydraulic system is connected with the parking brakecylinder 208 and the service brake master cylinder 212.

The hydraulic system 100 is arranged so that the power steering circuithas priority over all other hydraulic functions in the vehicle. For thispurpose, a priority demand valve 214 is provided; for design purposes,it is located in the steering unit 64. The steering unit 64 alsoincludes the steering valve 216 which controls energization of thesteering cylinder 68. The main valve 66, for design purposes, includesthe lift control valve 104 which controls the lift cylinder 26 and thetilt control valve 106 which controls the tilt cylinders 28 and 28'. Themain valve 66 also includes the auxiliary control valve 108 whichcontrols the auxiliary cylinder 218. It also includes an auxiliarycontrol valve 108' which controls an auxiliary cylinder, not shown. Forthe purpose of determining flow priority, the priority demand valve 214is located upstream of the steering control valve 216 and the main valve66. A pressure relief valve 222 is coupled between the priority demandvalve 214 and the steering control valve 216. A two-stage pressurerelief valve, also known as a hydrostat, is located, for designpurposes, in the main valve 66. The two-stage pressure relief valveincludes a secondary or main relief valve 224 which is controlled by aprimary or pilot relief valve 226 and by a primary or pilot relief valve228. A more detailed description of the hydraulic system will be givenbelow.

As shown in FIG. 12, the pump 60 has its inlet connected through aconduit or line 232 to the tank 74. The outlet of the pump 60 isconnected through a line 234 to the inlet port 233 of the prioritydemand valve 214. The priority demand valve 214 has a primary outletport 235 connected with a line 236 and a secondary outlet port 237connected with a line 238. The priority demand valve comprises a flowrestrictor or orifice 242 connected between the inlet port 233 and theprimary outlet port 235 and it includes a relief valve 244 connectedbetween the inlet port and the secondary outlet port 237. The reliefvalve 244 is biased closed by the bias spring 246 and by fluid pressurecommunicated through a pilot line 248; the valve is urged toward theopen position by the pressure communicated through a pilot line 252. Thepriority demand valve 214 is adapted to give priority to the flowrequirements of the power steering system through the primary outletport 235 to line 236; if the input flow to the priority demand valvethrough line 234 is greater than that to be allocated to the powersteering system, the excess flow is diverted through the relief valve244 to the secondary outlet port 237 and through line 238 for use byother hydraulic functions. For this purpose, the parameters of theorifice 242 and the relief valve 244 are correlated so that thefull-rated flow for power steering through the orifice 242 produces asufficient pressure drop thereacross to open the relief valve 244.Accordingly, the flow through the primary outlet port 235 into line 236will never exceed the full-rated flow of the power steering system andif the inlet flow from the pump exceeds that value, the excess flowsthrough the relief valve 244 to the secondary outlet and the line 238.For example, in the illustrative embodiment being described, the powersteering circuit may be rated for a maximum of five gallons per minuteand a maximum of 1000 PSI. It will be appreciated that the actual flowand pressure will be determined by the actuation of the steering controlvalve and the load imposed by the steering system. The flow produced tothe inlet port 233 of the priority demand valve 214 through the line 234depends, of course, upon the speed of the pump 60. As will be describedfurther below, in the example of this illustrative embodiment, operationin a low speed range of about 800 RPM is used for the power steering andtilt functions. This provides about five gallons per minute and thepressure may vary over a range up to 1000 PSI, depending upon load.Operation in an intermediate speed range of about 1200 RPM is used forthe auxiliary devices such as a load handling clamp; the flowrequirement may be about nine gallons per minute and the pressure mayrange up to 2000 PSI. In operation of the pump at high speed which maybe about 1800 RPM for the lift function, the flow may be about 20gallons per minute with pressures ranging up to 3000 PSI.

Power Steering Circuit

The power steering circuit comprises the steering control valve 216, thesteering cylinder 68 and it also includes a follow-up motor 254. Thesteering control valve 216 is a metering valve having an open centerspool adapted for bi-directional actuation from a neutral position. Thevalve is adapted to control the energization of the steering cylinder 68which is a double acting cylinder for actuating the dirigible vehiclewheels. The follow-up motor 254 has its rotor connected through amechanical follow-up linkage 256 with the valve 216 to null the valvewhen the dirigible wheels reach the commanded position. The primaryoutlet port 235 of the priority demand valve 214 is connected throughthe line 236 and a check valve 258 to the pressure inlet port 257 of thesteering control valve 216. A primary return line 262 is connected witha return port 259 on the control valve 216. A secondary return line 264is connected with a return port 261 on the control valve 216. The reliefvalve 222 is provided to prevent excessive pressure in the steeringsystem. For this purpose, it has its inlet port connected through a flowrestrictor 221 and a line 223 to the downstream side of a flowrestrictor 225 in the pilot line 248. The outlet port of the reliefvalve 222 is connected through a line 227 to the secondary return line264. Thus if the pressure at the outlet port 235 of the priority demandvalve 214 becomes excessive, the pressure relief valve 222 opens anddumps fluid to the return line 264. The steering control valve 216 hasprimary outlet ports 229 and 231 connected with opposite ends of thesteering cylinder 68 and follow-up outlet ports 239 and 241 connectedwith opposite ports of the follow-up motor 254.

Lift-Lower Circuit

The lift-lower circuit includes a lift control valve 104 which is ametering valve having an open center spool which is bi-directionallypositioned by the lift control lever 32. The lift control valve 104communicates with the single-ended lift cylinder 26 through a lift-lowercontrol valve 266 and a lowering flow control valve 268. The secondaryoutlet port 237 of the priority demand valve 214 is connected throughline 238 to a line 272 and thence through a flow restrictor 274 and line276 to the open center inlet port 277 of the valve 104. The open centeroutlet port 279 of the valve 104 is connected through a line 278 to thetilt control valve 106. The priority demand valve 64 also has itssecondary outlet port 237 connected with the control inlet port 282 ofthe lift control valve 104 through the line 238 and a line 284. Thecontrol outlet port 286 of the lift control valve 104 is connected withthe port 288 of the lift-lower valve 266. The lift control valve 104 hasa return line port 292 connected to a return line 294 which, in turn, isconnected through the return line filter unit 76 to the tank 74.

The lift-lower valve 266 comprises a ball-check 296 which is connectedbetween the port 288 and a port 298 for operation in the load liftingmode. The port 298 is connected with the port 302 of the flow controlvalve 268. The lift-lower valve 266 is provided with a flow-throughconnector 304 which is connected between the ports 288 and 298 foroperation in the load lowering mode. For the purpose of shifting theconnector 304 when operating in the lowering mode, the valve is providedwith pilot lines 306, 308, 312 and 314.

The lowering flow control valve 268 has a port 302 connected with theport 298 of the lift-lower valve 266. It also has a port 316 connectedwith the lift-lower single-ended cylinder 26. A check valve 318 and avariable orifice, constant flow valve 322 are connected in parallelbetween the ports 302 and 316.

Operation of the lift-lower circuit will be described in greater detailsubsequently.

Tilt Circuit

The tilt control valve 106 is a metering valve of the type having anopen center spool which is bi-directionally movable by the tilt controllever 34. The valve 106 communicates with the single-ended tiltcylinders 28 and 28' through a counterbalance valve 324. The tiltcontrol valve 106 has an open center inlet port 326 connected throughthe line 278 with the open center outlet port 279 of the lift controlvalve 104. Control valve 106 also has an open center outlet port 328which is connected through a line 332 to the auxiliary control valve108. The tilt control valve 106 has a control inlet port 334 connectedwith the secondary outlet port 237 of the priority demand valve 214;this connection extends through the line 238 and 272 and a flowrestrictor 336 and a check valve 338 in a flow control valve 342. Theflow control valve 342 also includes a pressure sensor in the form of aball-check 344 which will be described further below. The tilt controlvalve 106 has a control outlet port 346 which is connected to a port 348of the counterbalance valve 324. Another port 352 on the counterbalancevalve is connected through a line 354 to the tilt cylinders 28 and 28'at the rod-end thereof. The counter-balance valve 324 includes a checkvalve 356 and a relief valve 358 connected in parallel between the ports348 and 352 thereof. The tilt control valve 106 also includes a returninlet port 362 which is connected through a line 364 with the tiltcylinders 28 and 28' at the piston ends thereof. A return outlet port366 is connected to the return line 294. Operation of the tilt circuitwill be described in greater detail subsequently.

Auxiliary Circuit

The auxiliary control valve 108 is a metering valve having an opencenter spool which is bi-directionally movable by the auxiliary controllever 36. The auxiliary control valve 108 is adapted to control theenergization of the double-acting auxiliary cylinder 218. The controlvalve 108 has a control inlet port 372 which is connected with thesecondary outlet port 237 of the priority demand valve 214 through aflow control valve 374. This connection extends through line 238 to line272, and thence to flow control valve 374 which includes a flowrestrictor 376 and check valve 378 connected in series. The flow controlvalve also includes a pressure sensor in the form of a check valve 379.The auxiliary control valve 108 has a control port 382 which isconnected through a line 384 to one end of the cylinder 218. Theauxiliary control valve 108 includes a control port 386 which isconnected through a line 388 with the other end of the cylinder 218. Theauxiliary control valve 108 has an open center inlet port 392 which isconnected with the open center outlet port 328 of the tilt control valve106 through the line 332. The valve 108 has an open center outlet port394 which is connected with the auxiliary control valve 108' through aline 332'. The valve 108 has a return outlet port 398 which is connectedwith the return line 294. The valve also has a flow return inlet port402 connected with the outlet port of the pilot relief valve 228. Areturn outlet port 404 is connected with the return line 294.

Operation of the auxiliary circuit will be described in greater detailsubsequently.

The auxiliary control valve 108' is identical to control valve 108 justdescribed and it is connected in the hydraulic system in exactly thesame manner to control an auxiliary double-acting cylinder (not shown).Accordingly, the description of auxiliary control valve 108 isapplicable to control valve 108' and the description will not berepeated.

Two-Stage Relief Valve

With further reference to FIG. 12, the twostage relief valve will now bedescribed. As previously alluded to, this two-stage valve comprises amain relief valve 224, a pilot relief valve 226 and a pilot relief valve228. This two-stage valve is adapted to relieve the pressure at theoutlet port 237 of the priority demand valve 214 when it exceeds apredetermined operating value for different operating conditions. Inparticular, when operating in the lift mode, i.e. with the lift controlvalve 104 open, which calls for high speed pump operation, the pressureis to be limited, for example, to 3000 PSI. When operating in theauxiliary mode, i.e. with the auxiliary control valve 108 open with thepump operating at intermediate speed, the pressure is to be limited, forexample, to 2000 PSI. The main relief valve 224 has an inlet port 404connected with the outlet port 237 of the priority demand valve 214. Theoutlet port 406 of the valve 224 is connected through a line 408 to thereturn line 294. The relief valve 224 includes a pilot port 412 on oneside and a bias spring 414 and a bias port 416 on the other side. Thebias port 416 is connected through a pressure sensing line 418, a flowrestrictor 422 and the flow restrictor 274 to the pressure supply line272. The pilot relief valve 226 has an inlet port 424 connected with thepressure sensing line 418 and has an outlet port 426 connected to thereturn line 408. A pilot port 428 is connected on one side and anadjustable bias spring 432 is connected on the other side. The biasspring is adjusted, for example, to cause the valve to crack open at3000 PSI. The pilot valve 228 has an inlet port 434 connected with thepressure sensing line 418 and an outlet port 436 connected with thereturn inlet port 402 of the auxiliary control valve 108. The returnoutlet port 404 of this control valve is connected to the return line294. The pilot relief valve 228 has a pilot port 437 on one side and anadjustable bias spring 438 on the other side. The bias spring 438 isadjusted, for example, for a pressure of 2000 PSI.

Operation

The operation of the illustrative embodiment of the invention will bedescribed with reference primarily to FIGS. 4, 7, 8 and 12 through 16.It is noted that FIGS. 13, 14, 15 and 16 are basically the sameschematic diagram as FIG. 12 described above; however, each of FIGS. 13through 16 has heavy lines superimposed thereon to depict the fluid flowpaths for different operating conditions. A heavy solid line representsa hydraulic line under pump pressure, and a heavy dashed line representsa hydraulic line with return flow to the tank, as indicated in thelegend of each figure.

Idle Mode

Referring first to FIG. 12, the hydraulic system will be described inthe standby or idle condition with the pump running at low speed withouta load on the system. The pump motor 62 is started when the key switch82 is closed and the driver's seat is occupied with the drive selectorlever in forward, neutral or reverse so that the pump switch 96 is alsoclosed. In this condition, pump motor 62 is energized by the voltagecontrol circuit 86 for operation at a low speed, about 800 RPM. The pump60 delivers a flow ranging up to about five gallons per minute to theinlet port 233 of the priority demand valve 214. This value of flow doesnot cause sufficient pressure drop across the flow restrictor to openthe valve 244. Accordingly, the entire flow from the pump is deliveredthrough the outlet port 235 of the priority demand valve to inlet port257 of the steering control valve 216. With no steering effort appliedto the steering wheel, the control valve is in a neutral position andthe fluid flows through the open center of the valve to the outlet port259 thence through the supply lines 262 and 238 to the main valve 66. Inthe main valve, the flow passes through lines 272 and 276 and throughthe open center ports of lift control valve 104, tilt control valve 106,and auxiliary control valves 108 and 108' to the return line 294 andback through the filter unit 76 to the tank 74. Also, the pump suppliesfluid under pressure from the supply line 234 to the parking brakecylinder 208 to release the parking brake of the lift truck in readinessfor vehicle movement. In this idle mode of operation, the lift truck isin condition for operation in the steering mode or any of the otheroperating modes, as will be described subsequently.

Steering Mode

When the lift truck is operated in the steering mode, the hydraulicsystem is energized in the manner depicted in FIG. 13. In this operatingcondition, the pump 60 is operated at low speed and pump pressure issupplied to the steering control valve 216 as described above withreference to FIG. 12 for the idle mode. Also, pressure is supplied fromthe pump to the parking brake 208 as previously described. When thesteering wheel 18 is turned either left or right, the steering controlvalve 216 is displaced in a corresponding direction for energization ofthe steering cylinder 68. For example, for a left turn, the spool of thesteering control valve 216 is displaced so that the inlet port 257 isconnected with the fol- low-up port 239 and thence through the follow-upmotor 254 to the follow-up port 241 and thence through the outlet port231 to the right hand side of the steering cylinder 68. At the same timethe left hand side of the cylinder 68 is connected through the port 229to the return port 261 and thence to the return line 264. To carryoverflow from the steering control valve 216, the inlet port 257 isconnected through a flow restrictor to the outlet port 259 and thencethrough line 262 to the main valve 66. The return line 264 extends tothe tank 74 by way of a connection with the master cylinder 212 of thevehicle service brake to maintain the master cylinder supplied withfluid. Thus, the steering cylinder 68 is energized to turn the dirigiblewheels of the lift truck and the follow-up motor 254 is energized tonull the position of the steering control valve when the dirigiblewheels reach the position commanded by the steering wheel. It will beappreciated that operation of the steering wheel for a right hand turnresults in the opposite displacement of the steering control valve 216with consequent opposite energization of the steering cylinder 68. Thesteering control valve 216 is a metering valve and hence a higher rateof turning of the steering wheel results in a greater flow to thecylinder 68 and faster displacement of the dirigible wheels. Thepriority demand valve 214 is operative to supply the required flow tothe steering system, as a matter of priority over the other hydraulicfunctions, up to the full-rated flow of the steering system, such asfive gallons per minute.

In the event that the driver makes an extremely sharp turn in eitherdirection, the piston in the steering cylinder 68 may bottom out, i.e.reach its limiting position, so that the cylinder cannot acceptadditional fluid. This results in operation of the relief valve 222.When the piston of the steering cylinder 68 bottoms out, the fluidpressure in the line 236 will increase abruptly. This increased pressurewill appear at the primary outlet port 235 of the priority demand valve214 and will be communicated through the flow restrictor 225 and theline 266 to the inlet port of the pressure relief valve 222. When thepressure exceeds a preset value, for example, 1000 PSI, the relief valve222 will crack open and will relieve the pressure through line 227 tothe return line 264. The flow through the relief valve 222 is restrictedby reason of the flow restrictor 225 and the flow restrictor 221 to avalue of 0.2 to 0.3 gallons per minute, which is sufficient to maintainthe pressure below the preset value.

Lift Mode

When the lift truck is operated in the lift mode and the steering modecombined, the hydraulic system is energized as shown in FIG. 14. Thisoperating condition is obtained by moving the lift control lever 32 inthe rearward direction which, as described with reference to FIG. 5,displaces the lift control valve 104 in the lift direction and it alsocauses the speed control switch 112 to close. When the speed controlswitch 112 (see FIG. 5) is closed, the voltage control circuit 86 causesthe pump motor 62 to operate at full speed of about 1800 RPM.Accordingly, the pump 60 will deliver a flow ranging up to 20 gallonsper minute to the inlet port 233 of the priority demand valve 214. Aflow of about five gallons per minute through the flow restrictor 242will cause a sufficient pressure drop thereacross so that thedifferential pressure between pilot lines 248 and 252, combined with thebias spring 246, will cause the valve 244 to open. Accordingly, the flowdelivered from the pump to the inlet port 233 will be divided betweenthe primary outlet port 236 and the secondary outlet port 237 of thepriority demand valve. The steering system will be given priority withsufficient flow for operation thereof, up to five gallons per minute, sothat the driver may operate the steering system at any time. When thesteering wheel is operated for a left turn, for example, the steeringsystem will be energized as previously described with reference to FIG.13. Additionally, the flow delivered by the pump 60 in excess of thatsupplied to the steering system will be delivered to the main valve 66through the secondary outlet port 237 of the priority demand valve 214.With the lift control valve 104 displaced in the lift direction, thefluid flows from the outlet port 237 through lines 238 and 284 to theinlet port 282 of lift control valve 104 and thence through the outletport 286 to the port 288 of the lift-lower valve 266. From the port 288,the fluid flows through the ball-check 296 to the port 298 and thencethrough the ball-check valve 318 in the lowering flow control valve 268to the lift cylinder 26.

In the event that the lift control valve 104 is actuated in the liftmode until the piston bottoms out in cylinder 26, the main valve 66 willoperate in the relief mode. As discussed above, the two-stage reliefvalve, including the main relief valve 224 and the pilot relief valve226, is adapted to relieve pressure in the lift control system when thepressure reaches a preset value, for example, 3000 PSI. For thispurpose, the pilot relief valve 226 is adapted to crack open when thepressure on the pressure sensing line 418 reaches 3000 PSI. Note thatthe pressure sensing line 418 communicates through the flow restrictor422 and through the lines 276 and 272 with the line 284 at the inletport 282 of the lift control valve 104. When the pilot relief valve 226opens, it reduces the pressure at the bias port 416 of the main reliefvalve 224. Accordingly, the main relief valve is opened and the twovalves operate cyclically to maintain the supply pressure below thepreset value of 3000 PSI.

Lowering Mode

Operation of the system in the lowering mode will be explained withreference to FIG. 12. In order to lower the load, the lift control lever32 is moved forwardly. This displaces the lift control valve 104 to thelowering position; it also leaves the speed control switch 112 in theopen position, the same as with the lift control lever in neutral. Thus,the pump motor 62 is operated in its low speed range of about 800 RPMand the pump 60 will deliver a flow ranging up to about five gallons perminute. In this operating mode, the lift control valve 104 is positionedso that the port 286 is connected with the return port 292 which, inturn is connected through the return line 294 to the tank 74.Additionally, the pilot port 287 is connected with the port 286 andhence the return line 294. Further, the open center inlet port 277 isconnected with the open center outlet port 279 and thence through theopen center ports of tilt control valve 106, auxiliary control valve 108and auxiliary control valve 108' to the return line 294. In thiscondition, the fluid in the lift cylinder 26 is pressurized by the loadon the carriage. Accordingly, the fluid in the cylinder 26 flows throughthe variable orifice, constant flow valve 322 and through theflowthrough connector 304, which is shifted to the open position by thefluid pressures on pilot lines 306, 312, 314 and 308. By reason of theconstant flow valve 322, the lowering flow control valve 318 iseffective to cause lowering of the load at constant speed.

It will be understood that the priority demand valve 214 is operative inthe lowering mode of operation to supply the required flow for operationof the power steering system.

Tilt Mode

In operation of the system in the tilt-forward mode, the hydraulicsystem is energized as shown in FIG. 15. This operation is obtained bymoving the tilt control lever 34 in the forward direction. This, asdescribed with reference to FIG. 5, displaces the tilt control valve 106to the tilt-forward position. There is no speed control switchassociated with the tilt control lever 34 and consequently operation ofthe lever does not affect the motor speed control circuit 86. Hence, thepump motor is energized for operation in the low speed range at about800 RPM and the pump delivers a flow, ranging up to about five gallonsper minute, to the inlet port 233 of the priority demand valve 214.Assuming no steering, the flow from the primary outlet port 236 of thepriority demand valve passes through the ports 257 and 259 of thesteering control valve 216 and thence through lines 262, 238 and 272 tothe flow control valve 342. At this valve, the fluid flows from the line272 through the flow restrictor 336 and the check valve 338 to the inletport 334 of the tilt control valve 106. With this valve in thetilt-forward position, the inlet port 334 is connected with the outletport 362 which is connected through line 364 with the piston ends of thetilt cylinders 28 and 28'. The rod-ends of the cylinders 28 and 28' areconnected through line 354 to the inlet port 352 of the counter-balancevalve 324. The relief valve 358 remains closed until the supply pressureon line 364, as sensed by the pilot line to the valve 358 is sufficientto open the valve 358. When the valve 358 opens, fluid from the rod-endsof the tilt cylinders 28 and 28' is returned through the relief valve358 through the ports 346 and 366 of the tilt control valve to thereturn line 294 and thence to tank 74. It is noted that thecounter-balance valve 324 is operative to maintain a positive fluidpressure in the piston-ends of the tilt cylinders so as tocounter-balance the tendency of the mast or upright to rotate forwardlyunder the influence of a load on the carriage.

In the tilt-forward mode of operation, it is possible to have anoperating condition (not depicted in FIG. 15) in which a pump flowthrough lines 262 and 238 is greater than the flow through thecounterbalance valve 324. In this event, an increased pressure at theinlet port 334 on the tilt control valve 106 will be sensed through theball-check 344 and applied to the bias port 416 of the main relief valve224. This will cause the main relief valve 224 to open and the excessflow will be relieved through the valve and through line 408 to the tank74.

The tilt-backward mode is not depicted in FIG. 15 but will be describedwith reference to FIG. 12. The tilt-backward operation is obtained bymoving the tilt control lever 34 rearwardly; this displaces the tiltcontrol valve 106 to the tilt-backward position. The voltage controlcircuit 86 for the motor 62 is unchanged by this movement of the tiltcontrol lever and, if the other control levers are not actuated, themotor is operated in its low speed range, as described above withreference to the tilt-forward mode. With the tilt control valve in thetilt-backward position, the fluid from the pump 60 flows through thepriority demand valve 214 and through the steering control valve 216 andthence through lines 262, 238 and 272 through the check valve 338 to theinlet port 334 of the tilt control valve 106. The flow proceeds throughthe outlet port 346 and the ball-check valve 356 and through the line354 to the rod-ends of the tilt cylinders 28 and 28'. The piston ends ofthe cylinders are connected through line 364 to the port 362 of the tiltcontrol valve and thence through the port 366 and the return line 294 tothe tank 74.

Auxiliary Mode

When the lift truck is operated in the auxiliary mode, the hydraulicsystem is energized as shown in FIG. 16. This operating condition isobtained by moving the auxiliary control lever 36 in either the forwardor rearward direction depending upon the desired direction of movementof the auxiliary load handling device which, for example, may be a loadengaging clamp. As described with reference to FIG. 6, movement of theauxiliary control lever 36 in either the forward or rearward directioncauses the speed control switch 144 to close. When the auxiliary controllever 36 is moved forward, the auxiliary control valve 108 is displacedto its forward position and when the lever is moved rearward the valveis displaced to its rearward position. With the speed control switch 144closed, the voltage control circuit 86 causes the pump motor 62 tooperate at its intermediate speed range of about 1200 RPM. Accordingly,the pump 60 will deliver a flow ranging up to 9 gallons per minute tothe inlet port 233 of the priority demand valve 214. The pump flow willbe divided between the primary outlet port 236 and the secondary outletport 237 of the priority demand valve 214, as previously described. Theflow from the secondary outlet port 237 will be delivered through lines238 and 272 and through the flow restrictor 376 and the check valve 378to the inlet port 372 of the auxiliary control valve 108. With thecontrol valve 108 in its forward position, inlet port 372 is connectedwith the outlet port 382 which in turn is connected through line 384 toone end of the auxiliary cylinder 368. The other end of this cylinder isconnected through line 388 and valve ports 386 and 398 and thencethrough the return line 294 to the tank 74. When the auxiliary controllever is moved in the rearward direction, the auxiliary control valve ismoved to the rearward position and the auxiliary cylinder 368 isenergized for movement in the opposite direction.

In the event that the auxiliary control valve 106 is actuated in theforward mode until the piston bottoms out in cylinder 368, the mainvalve 66 will operate in the relief mode (not depicted in FIG. 16). Inthis case, the two-stage relief valve, which includes the main reliefvalve 224 and the pilot relief valve 228, is adapted to relieve pressurein the auxiliary control system when the pressure reaches a presetvalue, for example, 2000 PSI. For this purpose, the pilot relief valve228 is adapted to crack open when the pressure on the pressure sensingline 418 reaches 2000 PSI. Pressure sensing line 418 communicatesthrough the ball-check 379 with the inlet port 372 of the auxiliarycontrol valve 108. When the pilot relief valve 228 opens, it connectsthe pressure sensing line 418 through the valve ports 402 and 404 andthence through return line 294 to the tank 74. This reduces the pressureat the bias port 416 of the main relief valve 224. Accordingly, the mainrelief valve is opened and the two valves operate cyclically to maintainthe supply pressure below the preset value of 2000 PSI.

Although the description of this invention has been given with referenceto a particular embodiment, it is not to be construed in the limitingsense. Many variations and modifications will now occur to those skilledin the art. For a definition of the invention, reference is made to theappended claims.

What is claimed is:
 1. In a work vehicle of the type having at leastfirst and second hydraulic load circuits,said first circuit including afirst manually actuable selector member operatively coupled with a firstcontrol valve and a first hydraulic motor operatively connected with afirst utilization device and adapted to be energized through the firstcontrol valve, said second circuit including a second manually actuableselector member operatively coupled with a second control valve, and asecond hydraulic motor operatively connected with a second utilizationdevice and adapted to be energized through the second control valve, asingle positive displacement hydraulic pump, a changeable speed electricmotor connected with the pump, energizing means for energizing saidelectric motor, said second hydraulic motor being operably at a highervalue of hydraulic flow than said first hydraulic motor, said pump beingadapted to supply the hydraulic flow required by the second hydraulicmotor when the electric motor is operated in a high speed range and tosupply the flow required by the first hydraulic motor when the electricmotor is operated in a low speed range, the improvement comprising: apriority demand valve having an inlet port connected with the pump, aprimary outlet port connected with the first control valve, a secondaryoutlet port connected with the second contro1 valve, and normally closeddifferential pressure responsive valve means between the inlet port andthe secondary outlet port, said priority demand valve including meansfor supplying to the primary outlet port all of the flow required by thefirst circuit when the selector members are actuated individually ortogether with the pump running in either of said speed ranges, saidpriority demand valve including means incorporating a bypass check valvefor permitting flow from the primary outlet port via said first controlvalve to the secondary outlet port around said valve means when thepressure in the second circuit is less than that in the first circuitand including differential presure means for opening said valve meanswhen the inlet flow exceeds that required by the first circuit with saidfirst and second selector members actuated together, whereby the inletflow in excess of that required by the first circuit is supplied to thesecondary outlet port and thence to a second circuit, and speed controlmeans operatively connected with said energizing means and with one ofsaid selector members for energizing the electric motor for operation insaid high speed range when the second selector member is actuated andfor energizing the electric motor in said low speed range when only thefirst selector member is actuated, said speed control means includingregulating means for maintaining the speed of the electric motor withinfirst predetermined limits during operation in the high speed range andwithin second predetermined limits during operation in the low speedrange, whereby the first hydraulic motor is supplied with hydraulicfluid to operate the first utilization device regardless of whether thesecond selector member is actuated.
 2. The invention as defined in claim1 wherein,said electric motor is a series DC motor, said energizingmeans is a battery, and said regulating means includes means forapplying DC pulses a variable mark/space ratio to the motor and meansresponsive to the motor current for adjusting the mark/space ratio toregulate the motor speed within said predetermined limits.